The invention relates to a mechanical component consisting of solid bodies or parts of bodies, which are mobile relative to one another and are separated from one another by a fluid organic mass, wherein it is possible to vary the frictional forces acting between the bodies by changing the molecular order in the organic mass.
The motion of solid bodies within machines and the motion of a machine relative to a fixed base are, inter alia, determined by the friction which is applicable in each case between the bodies concerned. The fundamental distinction must here be made as to whether the bodies slide on one another or roll on one another. The magnitude of the sliding friction depends on whether the bodies slide directly on one another or are completely separated from one another by a lubricant. This is called dry friction in the first case and hydraulic friction in the second. So-called semi-hydraulic friction occurs if the lubricating film is incompletely formed. Sliding friction also takes place always in rolling bearings between the rolling elements and their guide elements.
As is known, the lubrication processes in machines can be classified in two groups. In hydrodynamic lubrication, the load-bearing capacity in a lubricating film is produced in the form of a gap of relatively large dimensions. The friction is then primarily determined by the temperature dependence of the viscosity of the lubricant. In the case of elasto-hydrodynamic lubrication, a very small lubrication gap is formed from an initially linear or punctiform contact of two elastic bodies. The flattening in the so-called Hertz contact region, together with the increase in the viscosity of the lubricant with pressure, has the result that the solid bodies moving relative to one another hardly touch directly or not at all. In existing practice, very high pressures, in the range of 1 to 40 kbar, are necessary to produce sufficiently high viscosities. By comparison, the pressures applied in hydraulic devices are much too low to enable a substantial change in the viscosity of the hydraulic oil and hence also the motion sequences. The pressure conditions in lubricants and in hydraulic oils have been described (Dubbel, Taschenbuch fur den Maschinenbau, [Pocketbook for Machine Engineering], Springer-Verlag, Berlin).
It is elementary to minimize the frictional losses in bearings by selecting a suitable lubricant. A large number of liquid lubricants--these are organic compounds in most cases--are nowadays in use. It is likewise elementary to ensure a high frictional force by selection of the materials, if a clutch effect or braking effect is to be achieved. The possibility, in principle, of changing the function of a mechanical component by varying the viscosity--for instance, changing the function from that of a slide bearing to that of a clutch--is provided by electro-rheology. In this case, the viscosity in layers of colloidal solutions is varied by means of an applied electric field (J. E. Stangroom, Electro-rheological Fluids, Phys. Technol., volume 14, pages 290-296 (1983)).
Some organic compounds do not pass directly from the crystalline phase into the isotropic-liquid phase on heating, but pass through one or more additional phases within clearly defined temperature ranges. These phases have anisotropic physical properties, as are observed in crystals, but are at the same time fluid like ordinary isotropic liquids. The phases formed by molecules of elongate shape are also described as a rod-like or calamitic phase. As distinct from the completely disordered isotropic phase, a long-range order of the orientation applies in this case. In the nematic phases (abbreviated as N) of hitherto known low-molecular compounds, the molecules can freely rotate about their longitudinal axis. Closely related to the nematic phase is the cholesteric phase which is formed by optically active elongate molecules or is obtained by addition of optically active compounds to nematic compounds. For purposes of the present invention, cholesteric phases are included in the term nematic phase. As a result of intermolecular interaction, parallel-aligned rod-like molecules can be assembled into layers and the latter can be arranged in space at always identical spacings. This layer structure is typical of the smectic phases. Different smectic phases can arise which differ by the arrangement of their components within the layers. The centres of gravity of the molecules within one layer can be arranged at random (for example in the S.sub.A phase and the S.sub.C phase) or regularly (for example in the S.sub.B phase). The phases have been designated approximately in the order in which they were discovered. Nowadays, smectic phases S.sub.A to S.sub.K are known. The features of such calamitic phases have been described (for example G. W. Gray, J. W. Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow (1984)). Liquid-crystalline phases can also be formed by disc-shaped compounds (so-called discoid phases). The discoidnematic phase here has the molecule arrangement which is the easiest to describe. In the so-called discoid-columnar phases, such molecules are combined in column-like arrangements as the result of intermolecular interactions. The features of discoitectic phases have been described, for example, in Mol. Cryst. Liq. Cryst. 106, 121 (1984). More recently, compounds have been disclosed which form so-called phasmidic phases, which are likewise thermotropic liquid-crystalline phases (for example J. Malthete et al., J. Phys. (Paris) Lett., volume 46, 875 (1985)). Thermotropic liquid-crystalline phases are also formed by polymers and their mixtures with low-molecular compounds (for example H. Finkelmann in Thermotropic Liquid Crystals, John Wiley & Sons, New York (1987) pages 145-170). The exploitation of the favourable viscosity within a single thermotropic liquid-crystalline phase for clock movements has already been described (European Patent 0,092,389).
It is known that the transitions between the Liquid-crystalline phases are pressure-dependent (for example G. M. Schneider et al., Physica 139 & 140 B, 616, (1986)). The dependence of the transition temperatures between the various phases are subject to the known Clausius-Clapeyron rules. In general, the existence ranges of the liquid-crystalline phases are shifted to higher temperatures by an increase in pressure. The order of the appearance of the phases remains unchanged in most cases, but it is possible that an additional pressure-induced phase arises. Thus, the transition temperatures of, for example the compound ##STR1## for normal pressure are S.sub.F -S.sub.C 160.degree. C., S.sub.C -S.sub.A 195.degree. C., S.sub.A -I 204.degree. C., for 250 bar S.sub.F -S.sub.C 171.degree. C., S.sub.C -S.sub.A 204.degree. C., S.sub.A -N 206.degree. C. and N-I 208.degree. C. It is also known that the intermolecular interactions which determine the viscosity change during such transitions.
The term phase transition includes, within the meaning of the mechanical component according to the invention, so-called pretransitional phenomena (described in: G. Vertogen, W. H. de Jeu, Thermotropic Liquid Crystals, Fundamentals, Springer-Verlag, Berlin 1988). These are changes in the molecular order and hence physical properties in the event of changes in pressure or temperature even before the phase transition is reached, for instance in the case of a pressure increase before the transition from nematic to S.sub.B is reached. The invention thus also comprises a mechanical component which changes its frictional force as a result of an induced change in the temperature or pressure in a fluid organic mass, if this mass has at least one enantiotropic thermotropic liquid-crystalline phase. In particular, it comprises such a component which is operated at temperatures which are 0.1.degree.-30.degree. C. above an enantiotropic transition from a liquid-crystalline phase into another liquid-crystalline phase or from the isotropic phase into a liquid-crystalline phase.
It is a requirement of the component according to the invention that a temperature or pressure difference is induced. The existence ranges of the phases concerned depend, in addition to the pressure, also on the temperature, so that changes in temperature during operation must be allowed for.
Conventional lubricants change their viscosity as a function of the temperature or pressure to such a small extent that very large temperature differences or pressures are required in order to obtain a favourable change in the frictional properties. For this reason, the applicability of this dependence to mechanical components--such as rolling bearings, rolling couplings and toothed gearings--with relatively small contact areas is limited in the case of high contact pressures and a high elasticity of the materials of the solid bodies concerned. Moreover, it has hitherto not been possible to use the electro-rheological liquids, because they tend to undergo sedimentation. In addition, the moving machine parts are subject to wear. In general, it is not easy to accomplish high electric field strengths between mobile, electrically conductive machine parts.